1. Field of the invention
This invention concerns the transmission of images, more specifically television pictures, by means of data, the flow of which is reduced by coding, and more specifically a process and system for differential coding and decoding of data for restricting propagation of errors in the event of wrong transmission.
Differential pulse code modulation (DPCM) of television signals to be processed or transmitted in digital form, in order to compress the flow of data involved, is already well known.
Such coding involves coding the difference between the value of a sample video signal and an estimate, or forecast, of this value, calculated from previous samples already coded. Differences, referred to as "forecasting errors", are classified in zones around nought (0), these zones being limited by threshold values. All differences of values comprised between two thresholds, corresponding to the same level of quantization are coded in accordance with a constant coded combination C.sub.i, corresponding to the zone of rank i situated between these two thresholds. Variations in the width of such zones, depending on their rank i higher than 0, determine what may be regarded as the compression characteristic, or quantization characteristic.
The problem to be solved by such coding is related to transmission errors.
If no precautions are taken, such methods are extremely sensitive to transmission errors, and a single such error can distort all subsequent decoded samples.
2. Description of the prior art
Various methods have been proposed to solve this problem.
One such method uses error-correcting codes for actual coding of quantized levels; these add redundancy bits to data bits before transmission. A problem with this technique is that after reduction of the flow of data to be transmitted by coding, an increase in flow is reintroduced by the addition of the redundancy bits, thereby defeating the P.
In another previously proposed method, a particular forecasting function is used to calculate the forecast, such as integration with loss, which prevents the error from being propagated beyond a certain distance.
A third proposed method consists essentially of analysing the signal received, in order to detect any change in its statistical properties caused by errors, thereby making it possible to correct the error later.
Finally, in another method, not only differences or errors in forescasting are coded, but also a forecasting error function, and the forecast itself.
These methods either increase data flow to correct errors, or do not allow enough correction to eliminate from reconstructed images any defects resulting from transmission error propagation.
French Pat. No. 2.408.945, and its certificate of addition No. 2.443.769, describe a method for reducing (coding) and expanding (decoding) of differential coding digital television signals, characterized by the choice of a quantization characteristic that, instead of being, symmetrical in relation to 0, as in earlier systems, non-symmetrical, as regards the number and values of quantization thresholds and levels. The certificate of addition more specifically concerns a method in which this quantization characteristic varies depending on the value of the forecast: the forecast variation range is divided up into forecasting zones, and each zone is assigned a quantization characteristic, which varies from one zone to the next, each such characteristic containing the same number of thresholds and levels of reconstruction, but the negative extreme levels differing from one forecasting zone to another.
This method has provided an improvement in images reconstructed from transmitted samples, since it doubles the number of possible reconstruction levels for the same number of transmitted bits; however, it does not prevent propagation of transmission errors.